Many forming die tools and processes for producing structures such as parts and panels from suitable sheet materials, which are sequentially formed superplastically and then diffusion bonded, are available. Unfortunately, the dies are subject to dimensional variations from manufacturing tolerances and temperature expansion, which make them difficult to design so that suitable pressure is applied by the dies at the right portions during fabrication of the structures to assure the desired diffusion bonding. This is especially true when curved or compound curved structures are being produced where a slight differential expansion, contraction or dimensional inaccuracy can result in improper mating of large areas of the dies.
Patents, such as U.S. Pat. No. 3,340,101 to Fields, Jr. et al.; U.S. Pat. No. 3,920,175 and U.S. Pat. No. 3,927,817 to Hamilton et al.; U.S. Pat. No. 5,069,383 to Cooper et al.; and U.S. Pat. No. 5,344,063 to Johnston et al. disclose methods of superplastic forming and diffusion bonding (SPF/DB) panel structures. However, generally the disclosed processes and tools are limited to either simultaneous forming and bonding or the use of maskants, also known as "stop off", in areas where bonding is not desired. Where a two-sheet SPF/DB process is desired, the use of maskants is possible. Unfortunately, maskants can affect the diffusion bond integrity of the sheets used during the process and must be very carefully handled. The maskants operate by preventing diffusion bonding from taking place in the areas where the maskant has been silk screened onto one of the adjoining sheet surfaces. It has proved to be very difficult to control the location of the maskant, particularly when panels have compound curves. If the maskant is not placed perfectly longitudinally, laterally and rotationally, the process fails, resulting in wrinkles, folds and other undesirable structural defects. One solution is shown in U.S. Pat. No. 5,285,573 to LeMonds et al., which discloses the use of diffusion bonding and then superplastic forming so that maskant can be avoided, or at least errors in maskant positioning can be discovered before the costs associated with superplastic forming are expended.
One solution to the problem of building two sheet SPF/DB structures is shown in U.S. Pat. No. 4,087,037 by Schier et al. and assigned to the McDonnell Douglas Corporation. The dies and tools disclosed by Schier et al. are primarily for making two sheet structures where a stiffener sheet is superplastically deformed into a rigid die to form reinforcing depressions and then the deformed sheet is diffusion bonded to a relatively planar face sheet. Any minor imprecision of the dies is overcome by using a flexible die sheet adjacent the face sheet to force the face sheet into diffusion bonding contact with the superplastically formed stiffener sheet. The relatively flexible die sheet is forced into the face sheet by the pneumatic expansion of a stainless steel bag trapped between the flexible die sheet and a backup die that only needs to approximate the shape of the face sheet. The flexible die sheet moves the face sheet into position for bonding and applies relatively even pressure to assure proper bonding. However, diffusion bonding temperatures are slightly higher than superplastic forming temperatures for the same alloy (between 1500 and 1750.degree. F.) so the forces applied by the bag through the flexible die sheet to the face sheet have a tendency to permanently deform the face sheet in areas where it is not supported by the rigid die. The resultant panel or structure has undesirable face sheet depressions. Since such structures are often used on aerodynamically critical exteriors of aerospace vehicles, dimples, valleys or other deformations are unacceptable, and the parts must be built up with smooth exterior skins connected with fasteners to the superplastically formed structures, which is not cost efficient, or made by the four sheet SPF/DB process as shown in Hayase, et al., U.S. Pat. No. 4,217,397, that is overkill for structures that will be lightly loaded. U.S. Pat. No. 4,426,032 by Leodolter, also assigned to McDonnell Douglas Corp., discloses an improvement to the Schier et al. tools that includes a face sheet die plate with flexible edges so that gas pressure can be used to force the die plate down on the face sheet.
Therefore, there has been a need for improved tooling that can be used to form two sheet SPF/DB parts.